Ethereum Classic, the smart contact platform, appears to be experiencing a block reorganization attack (reorg), according to blockchain explorer and mining pool, Etherchain. A reorg attack could potentially delay or reverse transactions on a blockchain. According to Etherchain, there was "a successful 51% attack on the Ethereum Classic network with multiple 100+ block reorganization." On Monday morning, the official Ethereum Classic Twitter account recommend that all exchanges and mining pools increase their confirmation times to over 400 blocks on ethereum classic withdrawals and deposits.
We can confirm that there was a successful 51% attack on the Ethereum Classic (#ETC) network with multiple 100+ block reorganization. We recommend all services to closely monitored the chain and significantly increase required confirmations.— Bitfly (@etherchain_org) January 7, 2019
Update: Coinbase confirms that it had detected reorg attacks on the Ethereum Classic network that included 88,500 ETC (~$460,000) in double spends.